WASHINGTON — In the final months of his life, Aaron Alexis complained of hearing voices talking to him through a wall and of microwave vibrations that he said entered his body and prevented him from sleeping. His delusional belief that he was being bombarded by extremely low-frequency radio waves escalated to the point that, before embarking on a murderous rampage that killed 12 people at the Washington Navy Yard, he left behind this note:

“Ultra-low frequency attack is what I’ve been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this,” read an electronic document FBI agents recovered after the shooting.

The FBI revealed that note from Alexis on Wednesday, along with peculiar notations on the shotgun he used, as evidence of a man in the throes of profound paranoia and delusions. Investigators have found no evidence that the September 16 shooting was inspired by a workplace conflict, saying he picked his victims at random and appears to have been driven by an unchecked mental illness.

The attack, which ended with Alexis shot dead by a police officer, came one month after he complained to police in Rhode Island that people were talking to him through the walls and ceilings of his hotel room and sending microwave vibrations into his body to deprive him of sleep. His shotgun, which he purchased two days before the shooting from a gun shop in Virginia, was etched with messages including “My ELF Weapon!” — an apparent reference to extremely low-frequency waves — and “End to The Torment!”

The ELF frequency range has historically been used for submarine communications, but some conspiracy theorists believe it allows for government monitoring and mind control of citizens, said Valerie Parlave, assistant director in charge of the FBI’s Washington field office.

Alexis, a 34-year-old former Navy reservist and computer technician for a government contractor, used a valid badge to get into the Navy Yard, entered a fourth-floor bathroom with a bag and emerged with a Remington shotgun with a sawed-off barrel and stock. He killed 12 workers, civilian employees and contractors ranging in age from 46 to 73, before being killed by a U.S. Park Police officer during a rampage and shootout that lasted more than an hour, the FBI said.

“There are indicators that Alexis was prepared to die during the attack and that he accepted death as the inevitable consequence of his actions,” Parlave said.

Surveillance video released by the FBI on Wednesday shows Alexis pulling his rental car into a garage, entering the building with a bag and then moving through a corridor with the shotgun, ducking and crouching around a corner and walking briskly down a flight of stairs. The video does not show the shots he fired.

A timeline issued by the FBI shows Alexis started the rampage on the building’s fourth floor and then moved down to the third and first floors. He ultimately returned to the third floor, where he was killed around 9:25 a.m. FBI Director James Comey has said there’s no evidence that Alexis shot down into the atrium despite earlier accounts from witnesses at the scene.

Alexis had started a job as a contractor in the building just a week before.

Although there was a “routine performance-related issue addressed to him” on the Friday before the Monday morning shooting, “there is no indication that this caused any sort of reaction from him,” Parlave said.

“We have not determined there to be any previous relationship between Alexis and any of the victims,” she said. “There is no evidence or information at this point that indicates he targeted anyone he worked for or worked with. We do not see any one event as triggering this attack.”

Defense officials have acknowledged that a lot of red flags were missed in Alexis’ background, allowing him to maintain a secret-level security clearance and access to a Navy installation despite a string of behavioral problems and brushes with the law.

He worked for The Experts, a Florida-based computer firm that was a Hewlett-Packard subcontractor. Hewlett-Packard said Wednesday that it was severing ties with The Experts, accusing the company of failing to respond adequately to Alexis’ mental problems.

The Experts said it was disappointed in the decision.

“The Experts had no greater insight into Alexis’ mental health than H-P, particularly given that an H-P site manager closely supervised him,” the company said in a statement.

At the Pentagon, Deputy Defense Secretary Ashton Carter said the department will review base safety procedures and the security clearance process.

“Bottom line is, we need to know how an employee was able to bring a weapon and ammunition onto a DoD installation, and how warning flags were either missed, ignored or not addressed in a timely manner,” Carter said.

Navy Secretary Ray Mabus has recommended that the department require that all police reports — not just arrests or convictions — be included in background checks.